1. Field of the Invention
The present invention relates to a method for imaging an exterior ship hull while underway or anchored using a remote sensing multi-beam sonar system array. More particularly, it relates to a multi-beam sonar array wherein a plurality of acoustic transducers are installed either in a shipping channel in a configuration that permits a ship's hull to be imaged while in transit through a channel or while the ship is stationary in an anchorage.
2. Description of the Prior Art
The capability to inspect the underside of freight ships and large vessels has always been of great concern for ports of entry, ports of call, and country port borders. It is well known that ships such as these have been frequently used to smuggle illegal drugs and contraband through ports of entry. This concern has been elevated in recent years as the focus has shifted to include nuclear, chemical, and/or biological agents, explosives, and any materials necessary to construct the foregoing. In many cases, these goods are smuggled through ports of entry on the underside of a vessel or ship by constructing a false bottom on the hull and storing these illegal or restricted items in the space created between the actual hull and the false bottom of the ship or vessel.
Inspection of a ship's interior spaces requires the use of scent dogs, inspection personnel and/or electronic devices, all of which are cost and labor intensive. Inspection of the underside of a submerged vessel is an even more difficult task, particularly since these ships, at a minimum, are more than several hundred feet long and draw between ten to twenty-five feet, depending on whether they are loaded with cargo.
One method of inspecting hulls, known in the prior art, is to send a diver to observe the underside of the ship through traditional visual means. U.S. Pat. No. 3,776,574 describes a process for the inspection of a submerged vessel whereby a diver employs a motion picture camera connected to a transmission cable to record the condition of the hull while submerged underwater. One drawback to this type of prior art system is that manpower is required to record every square inch of the submerged hull, thereby adding expense and time to the inspection process of the vessel. Other factors, such as environmental conditions can also have detrimental effects on the quality of the recordings and include water clarity, water temperature, and salinity. It should be noted that ideal visual imaging conditions almost never exist in marine port and docking facilities. Visual water clarity is typically measured in inches, thus allowing for divers to miss crucial areas of the hull during inspection dives. This prior art process is clearly inadequate and antiquated for today's needs in border protection, control, and inspection.
Advances in sonar technology have produced much better results than traditional visual inspection. Sonar technology uses sound echoes (acoustic reflected imaging) to produce an visual image of an object. A transducer converts electrical energy to sound energy and propagates sound pulses through the water in a wide angular plane. The pulses bounce off the intended object to be observed and the reflected pulse (echo) is recorded by a sensor coupled to the system. The data is transmitted to a processor which integrates the received signals to create and display a visual image. However, a limitation to sonar is the Doppler Effect which occurs with sound waves. The Doppler Effect is responsible for a shift in frequency of the sound wave of a moving object as it passes by. As an object is moving towards you or a sensor, the frequency of the sound wave appears higher, since the perceived wavelength is shortened thereby producing a higher pitched sound. When the object passes by you and continues to move away, the sound wave appears to stretch out, producing a lower frequency (perceived longer wavelengths), which in turn produces a low frequency sound. In underwater sonar, the Doppler Effect can attenuate sound waves and cause distorted imaging thereby playing havoc on the desired results. In addition, sound waves in the water column can be distorted by variations in suspended particles, salinity, density, temperature, surface and subsurface wave activity, and bottom features. To combat these conditions, the prior art devices involve mobile units which are manipulated across a surface of an observed object or towed units, which follow closely behind an observed object in order to minimize image distortion of the Doppler Effect and other annoying conditions.
U.S. Pat. No. 3,426,585 utilizes sonar technology to inspect the condition of submerged surfaces such as pilings. A transducer moves along a series of rods and about a periphery of a piling in a scanning pattern, transmitting sound pulses. The pulses are reflected back and a sensor of the transducer records the condition of the object. This type of transducer would not be suitable for inspecting objects such as large freight tankers. U.S. Pat. No. 6,317,387 describes a remotely operated vehicle (ROV) equipped with inspection cameras and position sensing equipment. The ROV is introduced into the water from the port and propels itself over the surface of a submerged hull to measure and record characteristics of the hull. This device does not employ sonar technology and is greatly inhibited by poor water quality in the same way that divers are inhibited during inspection dives. Furthermore, a remote operated vehicle requires extensive training, manpower and support to operate and to maintain the vehicle and is therein inadequate for today's needs.
U.S. Pat. No. 4,102,203 describes a process for inspection of a submerged surface of an underwater hull or vessel. This device utilizes a mobile transducer to inspect the underside of a ship, which is afloat. The transducer takes recordings, which are processed to create a visual image. The image produced by this prior art device is limited to a pictorial diagram and does not contain the high level of detail desired for a thorough and accurate representation of a ship's hull. Other devices are known in the prior art that employ transducers mounted on a platform or a cylindrical “fish” and towed behind a ship and used in bathymetry (the study of the depths of the ocean). These devices too are inadequate for today's needs since they are incapable of producing a clear image of the ship hull as it passes by the transducers.
There is a great need for a device that utilizes a sonar transducer array to produce real-time three dimensional, high resolution images that can be operated both in a stationary configuration and/or in a mobile configuration, and that allows for quick and accurate imaging of a ship's hull as it passes through a shipping channel or while it lies at anchor, yet is able to overcome all of the limitations associated with the use of prior art underwater sonar devices. The system should be able to render accurate images quickly so that human analysis can be conducted quickly such that suspicious vessels can be segregated from those not posing any terrorist risk or violating any laws of the sovereign nation's port of entry.